The present invention relates to a method of producing modified polyester moulded articles and to the moulded articles obtained thereby. The objective thereof lies in offering a method of producing moulded articles which, while still maintaining their low-temperature impact resistance or moist heat resistance, are also outstanding in their injection moulding and extrusion moulding properties and, furthermore, even when deformed do not undergo phase separation or fibrillation; together with the moulded articles formed thereby.
On account of their outstanding chemical and mechanical properties, polyesters have long been widely used in applications of various kinds such as fibres, films, industrial resins and bottles. These polyesters have in recent years come to be used under severe conditions, not just for motor vehicle and electrical/electronic components in particular, but also for industrial applications in general. In such applications, conventional polyesters have inadequate low-temperature impact resistance and they are susceptible to hydrolysis under high humidity and high temperature conditions, so there have been restrictions on their use.
For the improvement of the low-temperature impact strength of polyester resins, there has to some extent been employed the incorporation of core/shell type impact absorbing agents in which a rubber or the like is used as one component, or the incorporation of a modified polyolefin having functional groups which react with the polyester terminals.
On the other hand, in regard to the hydrolysis of polyesters, it has long been known that this problem is promoted by terminal carboxyl groups and, in order to overcome it, there are methods such as suppressing the terminal carboxyl group concentration by low-temperature melt polymerization and solid phase polymerization, or by adding a compound which reacts with the terminal carboxyl groups such as alkaline compounds, diglycidyl terephthalate, polycarbodiimide, ethylene carbonate and imidazoline compounds. Of these methods, some have already been employed on a practical basis. However, with an approach based on the method of polymerization, productivity is inevitably sacrificed to some extent, while in the case where a reactive compound is added, problems arise such as the adverse effects caused by the low molecular weight materials produced as a result of the reaction or decomposition at the time of melt fabrication, and so further improvements are demanded in relation to applications where used under more severe conditions.
In order to resolve such problems, there is known the incorporation of a polyolefin which has been modified by means of glycidyl groups or the like, so as to reduce the terminal carboxyl groups concentration by reaction therewith and, furthermore, so as to reduce the viscosity at the time of melt fabrication. In such circumstances, if, for example, an ethylene-alkylÿacrylate-glycidyl terpolymer is used in order to lower the glass transition point at the same time, or to further increase the compatibility with the polyester, the acrylate partially decomposes during the melt fabrication and carboxyl groups are newly produced, which then promote the polyester hydrolysis. In order to prevent such a problem, the modified polyolefin has to be restricted to an ethylene-glycidyl copolymer which is not a terpolymer containing alkyl acrylate. However, when an ethylene-glycidyl copolymer is used, while there is reaction with the molecular terminals of the polyester, there is inadequate compatibility between the polyethylene regions and the polyester, and phase separation occurs when the moulded article is subjected to bending or other such repeated deformation during use. In certain circumstances, fibrillation occurs, so that not only is the appearance of the moulded article impaired but the product performance is also lowered, and so a resolution of this difficulty has been demanded.
As a result of a painstaking investigation aimed at obtaining moulded articles which, while maintaining their low-temperature impact resistance and moist heat resistance, are also resistant to flexural fatigue, the present inventors have discovered that this objective is realised by incorporating an ethylene copolymer at the same time as the glycidyl copolymer, and it is on this discovery that the present invention is based.
The present invention is a method of producing modified polyester moulded articles which is characterized in that, when producing moulded articles from a modified polyester composition comprising a polyester composed of an acid component chiefly consisting of aromatic dicarboxylic acid and a glycol component chiefly consisting of aliphatic diol, and a modified polyolefin, there is incorporated an ethylene-acrylate copolymer; together with the moulded articles produced thereby.
In the present invention, xe2x80x98polyesterxe2x80x99 is a polyester composed of an acid component chiefly consisting of aromatic dicarboxylic acid and a glycol component chiefly consisting of aliphatic diol. As examples of the xe2x80x98aromatic dicarboxylic acidxe2x80x99, there are terephthalic acid, naphthalene dicarboxylic acid, isophthalic acid, diphenyl dicarboxylic acid, diphenyl ether dicarboxylic acid, diphenyl sulphone dicarboxylic acid and diphenyl ketone dicarboxylic acid. As examples of the xe2x80x98aliphatic diolxe2x80x99, there are ethylene glycol, trimethylene glycol, tetramethylene glycol and hexamethylene glycol. Here, xe2x80x98chieflyxe2x80x99 is not particularly restricted but broadly means 80 mol %. Other acid component or hydroxy component may be copolymerized within a range below 20 mol %. Again, there can also be used a polyfunctional compound with three or more functional groups, within a range such that the polyester remains substantially linear. Amongst the aforesaid polyesters, polyethylene terephthalate, polyethylene naphthalate and polybutylene terephthalate are preferred. Such polyester is produced by a conventional melt polymerization method but, where required, may also be based on a solid phase polymerization method.
In the present invention, xe2x80x98modified polyolefinxe2x80x99 is a polyolefin which has undergone graft- or co-polymerization with an unsaturated epoxide or derivative thereof. A polyolefin which is a graft- or co-polymer of an unsaturated acid or anhydride or other derivative thereof, although it is a modified polyolefin nevertheless lies outside the scope of this invention. The reason for this is because the carboxyl groups produced as a result of reaction with the polyester will promote the hydrolysis of the polyester. As specific examples of modified polyolefins which have been modified by means of an unsaturated epoxy, there are ethylene-glycidyl copolymer, propylene-glycidyl terpolymer, butylene-glycidyl copolymer and the like. Of these, the terpolymer has inferior heat resistance compared to the copolymers, so the copolymers are preferred and ethylene-glycidyl copolymer is particularly preferred. These are polymerized by conventional methods.
In the present invention, the proportion of the polyester composition represented by said modified polyolefin will differ depending on the application but, normally, it is no more than 30 wt % and preferably no more than 20 wt %.
In the present invention, xe2x80x98ethylene-acrylatexe2x80x99 copolymer is a copolymer chiefly comprising ethylene units in which there are also alkyl acrylate ester units. The ethylene content is at least 60% and preferably at least 70%. Furthermore, the alkyl ester will have no more than 8 carbons and preferably no more than 4 carbons, specific examples being the methyl, ethyl and butyl esters. Of these, copolymers such as ethylene-methyl acrylate and ethylene-butyl acrylate can be cited as preferred examples.
In the present invention, the amount of ethylene-acrylate copolymer is preferably from 3 to 40 wt % in terms of the modified polyolefin. If there is less than 3 wt %, then there is little effect in terms of preventing phase separation, while if there is more than 40 wt % the heat resistance of the polyester may be lowered. A part of the ethylene-acrylate copolymer may undergo thermal decomposition and the moist heat degradation of the polyester may be promoted as a result of the carboxyl terminal groups produced, so it is preferred that as little as possible be employed. From 5 to 35 wt % is further preferred.
The reason why the flexural fatigue resistance is enhanced while maintaining the low-temperature impact resistance and the moist heat degradation resistance by the joint use of an ethylene-glycidyl copolymer and an ethylene-acrylate copolymer along with the polyester, is not clear. However, it is assumed that, since ethylene-acrylate copolymers have inferior thermal stability than ethylene-glycidyl copolymers, but have superior heat resistance compared to ethylene-acrylate-glycidyl terpolymers, by combining an ethylene-glycidyl copolymer and an ethylene-acrylate copolymer, the moist heat resistance is simultaneously improved while still maintaining the low-temperature impact resistance and the compatibility at the ethylene-acrylate-glycidyl terpolymer level.
There may be freely included in the composition of the present invention, known antioxidants, thermal decomposition preventives, ultraviolet absorbers, hydrolysis resistance improvers, colouring agents (dyes and pigments), antistatic agents, electro-conductors, crystallization nucleating agents, crystallization promoters, plasticizers, ready slip agents, lubricating agents, release agents, flame retardants, flame retarding auxiliaries and reinforcing fillers (glass fibre, glass beads, carbon fibre, aramid fibre and the like) within a range such that the characteristics of the present invention are not impaired. For example, in the case of a reinforcing filler, it is necessary to take care that the surface treatment agent thereof or the converging agent be selected such that the objectives of the invention are not impaired.
In the present invention, xe2x80x98moulded articlexe2x80x99 refers to an injection moulded article or extrusion moulded article. As examples of injection moulded articles, there are ordinary electrical/electronic components and automobile components. As examples of melt extruded articles, there are fibres, films, sheets, tubes and pipes, etc.
In regard to the composition of the present invention, carrying out the moulding with the polyester and modified polyolefin and/or ethylene-acrylate copolymer already melt-blended will have a considerable effect in terms of preventing phase separation but, from the point of view of the heat resistance and moist heat resistance of the polyester moulded articles, the less the thermal history the better, so it is preferred that direct extrusion moulding be carried out using a screw of a type provided at its tip with a Dulmage or Maddock mixing zone. Of course, it is also possible to supply each component to the moulding machine and then carry out direct injection moulding.